The present invention relates to a can seam inspection device and, more particularly to an improved optical seam inspection device which provides increased inspection performance due to improved illumination methods.
The purpose of a can seam inspection device is to accurately measure the component parts of a double seam as is typically found around the top of a normal soup-type can. Traditionally, seam measurements were taken by tearing the seams down into their components parts and measuring them with a seam micrometer. This method has been used for many years and is still used regularly in canning plants having low product volume. Seam micrometers are limited, however, in that the positioning of parts therein may vary and thus affects readings, that part measurements vary from inspector to inspector due to slight and varying deformations of the part caused by personal differences in micrometer pressure. Seam micrometers are also time consuming and not practical for high volume production inspection.
An improvement on the traditional method has come in the form of video-based can seam inspection devices which display magnified images of specially prepared seam cross-sections on a video screen or computer monitor where they are measured with video cross-hairs or cursor lines that have been calibrated to measurement units. A video-based can seam inspection device consists mainly of three groups of components: optical, electrical, and mechanical. The optical group comprises a video camera, a magnifying lens, a light source, and a mirror, and in some cases a fiber optic light pipe. The camera and magnifying lens are located toward the back of the device and point directly toward the front of the device where the mirror is located. The mirror is mounted vertically and is angled 45 degrees with respect to the front of the device so that it allows the camera to look 90 degrees to the side of the device. The optical axis of the camera and magnifying lens are coincidental and also pass through the center of the mirror. A small light source is located near the optical axis such that it casts light in a direction essentially parallel to the optical axis and toward the mirror. In some conventional can seam inspection devices a fiber optic light pipe is used to convey the light from a light source to a point near the optical axis instead of placing the light itself near the axis. The electrical group comprises a power supply which powers the light source, and in some devices, also the camera. The mechanical group comprises a base, a mirror mount, a can platform, and an enclosure. The camera and magnifying lens are attached to the base toward the back of the device. The mirror mount holds the mirror in a fixed orientation protruding out slightly from the front of the base and is attached to the front of the base at a height centered about the optical axis of the device. The can platform is also attached to the front of the base in an orientation which is essentially horizontal and parallel to the optical axis and at a height at which a can seam sample is visible to the camera when a can is placed on the platform. The enclosure enshrouds all components except for the can platform, the mirror mount and mirror. It serves to protect the internal components and also shields the camera from stray light. In practice, to perform measurements, a prepared can is placed on the platform so that the seam is visible to the camera. The light source casts predominantly direct light on the seam via the mirror thus illuminating the seam. The illuminated seam reflects light back via the mirror to the magnifying lens and camera which picks up the enlarged image and transmits it to either a video screen or a computer monitor where it is measured with video cross-hairs or line cursors.
The conventional video-based measurement method has clear advantages over traditional methods since it does not measure through contact of the part, so is not subject to the same problems of part positioning or measurement pressure with its resulting part deformation. Measurement accuracy is improved as is inspection efficiency.
Although the conventional video-based can seam inspection device has clear advantages over traditional methods, there are several drawbacks which stem from the prior art's dependence on predominantly direct illumination. These drawbacks affect the accuracy of the device and its use over a wide range of materials. An ideally prepared seam is perfectly flat across its surface, uniformly reflective and composed of metal. Although direct illumination is very appropriate in this situation, it becomes less appropriate the further the seam departs from ideal. Due to the quality of the saws used in the preparation of seam samples, many prepared seams have rounded or nicked edges that make the seam appear smaller under direct illumination. Still other seams, containing regions with disparate reflectivities, such as plastic or composite seams, when viewed under direct illumination, lack enough definition to reveal key edges of the seam components, thus can not be measured at all. Also, in situations where a seam must be analyzed for attributes other than dimensional, as in a visual inspection, direct illumination severely limits visual cues that reveal surface detail and texture.
It is therefore the object of the present invention to provide an improved video-based can seam inspection device which utilizes improved illumination methods that both provide increased measurement accuracy and add the capability of the device to effectively measure non-metallic seams.